Solar-photocatalytic of zinc oxide for mineralization of phenol
Nik Noor Athirah Binti, Nik Yusoff
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There are increasing concerns on the significant of phenols as an organic contaminant from industrial wastewater such as pesticides, coal conversion, polymeric resin, petrochemical industry, pharmaceutical and oil refinery industries. Phenols can be threatening to human being and ecosystems due to its biorecalcitrant and acute toxicity behavior. There are some limitations for phenols treatment via conventional wastewater treatment such as in biological treatment, a longer time which is required; membrane treatment is expensive and another pollutant are generated via activated carbon treatment. Zinc oxide (ZnO) utilization in Advanced Oxidation Process (AOP) via solarphotocatalytic process was a promising method for treating wastewater containing phenol. The photocatalytic degradation of phenol was investigated with zinc oxide (ZnO) as photocatalyst under solar light irradiation. Operating parameters such as initial phenol concentration, catalyst loading, pH, effect of aeration, H2O2 dosage and effect of solar light irradiation were investigated. The low initial concentration of phenol indicates more efficient photocatalytic degradation. The optimum catalyst loading to provide sufficient active site for the photocatalytic activity is 0.6 g. While, the optimum pH condition is in acidic condition as it show a better performances than in alkaline condition. The photocatalytic activity improved with aeration and the photodegradation rate is 14.325 mg L-1 h-1 . Besides that, the addition of 0.1 M H2O2 also enhanced the degradation of phenol. The results obtained fitted well with Langmuir-Hinshelwood kinetic model. Analysis of UV-VIS and chemical oxygen demand (COD) attested the complete degradation of phenol concentration and possibility for mineralization. The environmental friendly ZnO photocatalyst semiconductor was synthesis by precipitation (ZnO-P), hydrothermal (ZnO-H) and sol-gel (ZnO-S) method. The morphologies of the photocatalyst were observed by SEM showed the morphology of ZnO-P and ZnO-H are pseudo-spherical shape with sizes of 20 nm until 130 nm. While, an irregular shape with sharp edges was observed for ZnO-S. The particles sizes of 110-400 nm were obtained for ZnO-S. The results from XRD analysis interestingly indicate all the characteristic peaks observed in synthesized ZnO are in a good agreement with the pure ZnO standard pattern taken from the Joint Committee of Powder Diffraction Standard (JCPDS) card No. 36-1451. The XRD patterns of all photocatalyst are the same with different intensity indicates different crystallite sizes. Particularly, the strongest characteristic peaks were described at 2θ 36.24 °, 36.31 °, and 36.32 ° for ZnO-P, ZnO-H and ZnO-S, which correspond to plane (1 0 1). The peak indexed as hexagonal with space group P63mc (186). Thermal analysis suggested the ideal calcinations temperatures are within range of 350 °C until 400 °C. The destruction of hydrocarbonate (OHˉ and CO 3 2ˉ) takes place at temperature 370 °C and 73.08 % of ZnO-P, 74.52 % ZnO-H and 72. 41 % ZnOS weight left. The decomposition of the ZnO precursor was complete at this temperature and can be considered as the optimum calcinations temperature for synthesized process. No further weight loss was observed from 430 °C until 800 °C. This plateau indicates the formation of the ZnO as a decomposition product. The comparison of photodegradation showed that the photocatalytic performances of all five tested photocatalyst could be arranged as ZnO-C > ZnO-S > ZnO-P > ZnO-H > TiO2. The finding of this study was described in Langmuir-Hinshelwood model. The apparent rate constant is proportional to the efficiency of the photocatalyst. Further research to evaluate ZnO photocatalyst would be of great help in developing the semiconductor solar-photocatalytic treatment which are currently still in an experimental phase worldwide. Hence, pilot plant design can be a good start to study the application of ZnO as photocatalyst in real wastewater treatment.